When crafting your project's electrical system architecture
it would be helpful to be aware of the marketplace for
to which avionics suppliers deliver most of their products.
99.9% of all light aircraft built feature an architecture
that has not changed materially in 40+ years when the
avionics bus and master switch was added to protect
transistors from will-o'-the-wisp spikes.
Those same aircraft are largely owned and operated
by individuals who know little if any more about their
airplanes than they do about their cars. Further, even
if they knew and understood a great deal about them,
they would not be permitted by regulation to make
any meaningful improvements upon them. Phrases
like "failure tolerance", "meeting design goals",
and "artful preventative maintenance for maintenance
of system reliability" might just as well be spoken
in Swahili.
Now we have the starry-eyed entrepreneur who
tailors a product for best-fit into spam-can/
FAA-trained aviation. Not only is the product marketed
and sold as the greatest accessory since sliced bread,
it may include a suite of "back up" features designed
to ward of a host of failures uncomfortable to contemplate.
Internal back up batteries are popular. Keep in
mind that internal batteries MIGHT make sense in
the spam-can/FAA-trained world of flight . . . but
we're OBAM aviation. We can craft power sources that
are exceedingly robust. In fact so robust that
flight-critical systems are more likely to become
unavailable due to internal failure than for lack of
ship's power.
Staying true to our design goals for failure tolerance:
If your are PLANNING for any accessory to be critical
to comfortable completion of flight, then your plan-B
for failure of that device needs to go BEYOND any
need for ship's power. If you have a system like Z-13/8
(and assuming further that you exercise due diligence for
battery maintenance) then the probability of the E-bus
ever becoming un-powered in flight is on the same
order as prop bolt failure.
At the same time, probability of some flight critical
system going down for for reasons OTHER than power
failure is decidedly higher.
With that reasoning in mind I'll suggest that fussing
over the optimum back-up battery installation for any
accessory is a distraction from the real task of
crafting a failure tolerant system.
If it were my airplane, I'd go for Z-13/8, ditch the
internal back-up batteries and figure out how I'm
going to comfortably deal with a failure of the LEDs
that light up the screen or a rate sensor that goes
south, etc. etc.
Bob . . .
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